eWLP technology is a semiconductor device packaging technology in which a multiplicity of device packages having semiconductor dies or chips embedded therein are co-formed with one another as part of a single “wafer” of moldable material and then transformed into individual packages by dicing or singulating the wafer. In addition to semiconductor dies or chips, various types of passive electrical components and optical elements may also be embedded in the wafer. The process of forming eWLP packages commonly begins with mounting a multiplicity of semiconductor dies on an adhesive tape base. A robotic pick-and-place machine is commonly employed in the mounting step. Next, a layer of molding compound, such as a liquid polymer, is applied to the dies and to the adhesive tape base, thereby embedding, or potting, the dies within the molding compound. The molding compound is then cured to harden it. The resulting assembly is analogous to a wafer of the type traditionally employed in semiconductor fabrication in that the assembly is singulated in a later step of the process. For this reason, such an eWLP assembly is sometimes referred to as a wafer.
The tape base is removed from the assembly, exposing the front side of the assembly. The molding compound surface on the back side of the assembly is ground down until the assembly has a target thickness. Next, a metal layer is applied to one or both surfaces of the assembly by, for example, metal sputtering, electro-plating, or both in sequential order. Each metal layer is then photolithographically patterned to form a redistribution layer (RDL) that defines electrical signal paths. In some types of eWLP processes, arrays of solder balls are formed on the RDL. The assembly is then diced into individual eWLP packages, each containing one or more semiconductor chips.
Optoelectronic devices or modules having eWLP packages are known. Optoelectronic modules, such as optical transmitter and receiver modules, for example, are used in optical communications systems and sensor systems. In the case of an optical communications system, an optical transmitter can convert electrical signals that are modulated with information into optical signals for transmission over an optical fiber. An optoelectronic light source, such as a laser diode, performs the electrical-to-optical signal conversion in the optical transmitter. An optical receiver can receive the optical signals transmitted over the optical fiber and recover the information by demodulating the optical signals. An optoelectronic light detector, such as a photodiode, performs the optical-to-electrical signal conversion in the optical receiver. The functions of optoelectronic modules in sensor systems are very similar, with an emitting device (e.g., a laser diode) performing the electrical-to-optical conversion and a receiving device (e.g., a photodiode) performing the optical-to-electrical conversion. Additional integrated circuits (ICs) might be included in the eWLP package for controlling the system or processing data and signals in the system.
The optoelectronic light sources, receivers and/or ICs incorporated into the eWLP packages have front side and/or back side contacts on them. The front side contacts become accessible when the adhesive tape is removed from the front side of the wafer. Any back side contacts, however, are typically encapsulated within the hardened molding compound, and therefore are not easily accessible. One way to access the back side contacts is to form thru-silicon vias (TSVs) or thru-mold vias (TMVs) in the bulk material of the chips or in the mold material, respectively, to create electrical pathways from the front side of the wafer to the back side of the wafer. Electrical connections (e.g., bond wires) may then be used to connect the ends of the vias disposed on the back side of the wafer to electrical contacts disposed on the back sides of the chips. The manner in which such connections are made within the wafer affects manufacturing economy.
Accordingly, it would be desirable to provide eWLP methods that allow back side electrical and/or thermal connections to be easily and economically made.